Time-domain systems are important analytical tools for measuring properties of an object. Recently, terahertz systems known as terahertz time-domain spectrometers (THz-TDS) have been developed. These systems often use visible to near-infrared laser pulses each lasting only 10 to several hundred femtoseconds to electromagnetic pulses (“T-rays”) that each last for about a picosecond. T-rays can be transmitted through various objects, using an imaging system of lenses and mirrors to focus or collimate the T-rays. As the T-rays pass through the object under test, they are typically distorted. These changes in the T-ray signals can be analyzed to determine properties of the object. Materials can be characterized by measuring the amounts of distortion-from absorption, dispersion and reflection-of the T-rays passing through to a detector. A digital signal processing system takes the digitized data from the THz detector and analyzes the data in either the spectral or temporal domain.
Because many compounds change T-rays in characteristic ways (e.g., absorption or dispersion), molecules and chemical compounds show strong absorption lines that can serve as “fingerprints” of the molecules. T-ray spectroscopy can distinguish between different chemical compositions inside a material even when the object looks uniform in visible light. A terahertz sensor for instance can be employed to measure caliper, moisture, and basis weight of paper whose thickness is similar to the wavelengths of T-Rays.
The precision of amplitude and phase measurements in time-domain (terahertz) spectroscopy (THz-TDS) is often limited by noise in the system. It has been demonstrated that the dominant types of noise present in THz-TDS are often time base and amplitude jitter characterized by pulses traveling through the same material (or air) which reach the detector at slightly different times and with slightly different amplitudes due to fluctuations in environmental parameters (e.g., temperature fluctuations or vibrations) or hardware errors (e.g., encoder errors in the delay line). In some specific THz-TDS systems, jitter makes a significant contribution to the noise and therefore limits the measurement precision of the system. In other THz-TDS systems, it is the multiplicative noise (i.e., amplitude noise), which comes primarily from the laser that is the main source of imprecision.
Prior art endeavors to reduce the adverse effects of jitter focused on controlling environmental parameters such as temperature and minimizing vibrations. In addition, improving the quality of the delay stage was also an important consideration. For example, US Patent Application Publication 2010/0007955 to Beselt describes a large amplitude high frequency optical delay that is particularly suited for use in a scanning terahertz sensor system that employs optical fibers that serve as launching and returning optics.